Method for producing TDI-trimerisates with high purity

ABSTRACT

The invention relates to a novel method for producing polyisocyanurates comprising isocyanate groups, based on 2,4- and 2,6-toluylene diisocyanate (TDI) and to the use thereof in coating compositions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of U.S.application Ser. No. 14/774,305, filed Sep. 10, 2015, which is anational stage application (under 35 U.S.C. § 371) of PCT/EP2014/054434filed Mar. 7, 2014, which claims the benefit of European Application No.13158688.5 filed Mar. 12, 2013, all of which are incorporated herein byreference in their entirety.

The invention relates to a novel method for producing polyisocyanuratescomprising isocyanate groups, based on 2,4- and 2,6-toluylenediisocyanate (TDI) and to the use thereof in coating compositions.

The production of polyisocyanurates comprising isocyanate groups hasbeen known for a long time and described in a large number ofpublications and patents (Houben-Weyl, Methoden der organischen ChemieVolume 8, p. 136ff, Georg Thieme Verlag Stuttgart 1952; H. Wagner, H. F.Sarx, Lackkunstharze 5th Edition, page 153ff, Carl Hanser Verlag Munich1971; DE-A 4 428 107, U.S. Pat. No. 2,993,870; DE-C 1 201 992; DE-A 2452 532; J. prakt. Chem. 336, p. 185 to 200, 1994). Both trimerisatesbased on aliphatic and aromatic diisocyanates are used universally ascoating raw materials and as polyurethane elastomers and polyurethanefoams.

Commercial products are nowadays produced by trimerising industriallyavailable mixtures of 2,4-TDI and 2,6-TDI in suitable organic solvents,with catalysis by means of phenolic catalysts comprising dialkylaminogroups (Mannich bases), to almost complete conversion and thendeactivating the catalyst by addition of acid-reacting substances or byreaction with alkylating agents. This deactivation is necessary becausethe products are otherwise not stable, which manifests itself by a fallin the NCO content and an increase in the viscosity over time.

For reasons of industrial hygiene, low-monomer trimerisate types aretoday preferred as products. These products are produced either byseparating off excess monomer by distillation after the trimerisationreaction has taken place, or by controlling the trimerisation reactionto correspondingly high conversions, until the monomer has reacted asfar as possible to higher oligomeric isocyanurates. The latter method issuccessful in particular when the diisocyanates used carry twosignificantly different reactive isocyanate groups—as in the case of2,4-toluylene diisocyanate. Corresponding products comprising solventcan thus be produced with a content of monomeric TDI (sum of theisomeric toluylene diisocyanates) of <0.5% (e.g. Desmodur® IL,commercial product of Bayer AG, 50% strength in butyl acetate, NCOcontent: 8.0%).

However, the polyisocyanates of the prior art produced in this mannerhave the fundamental disadvantage that, as a result of their production,they are always contaminated by the stopped catalysts, the contentthereof being greatly dependent on the reactivity of the toluylenediisocyanate used or on the amount of catalyst required in a particularcase. This contamination by the stopped catalysts leads to more rapidyellowing and ageing of the polyisocyanates and of the coatings producedtherewith.

The reason for the increased catalyst consumption and the resultingdiscolouration of the polyisocyanates and of the coatings producedtherewith is in particular the use of TDI of insufficient purity. Therehas therefore been no lack of attempts to provide TDI grades with whichlighter-coloured aromatic polyisocyanates which are more stable toageing can be produced. For example, EP 1 413 571 describes a processwith which a product fraction having a TDI content of at least 99.5% andless than 200 wt·ppm solvent and/or chlorinated aromatic hydrocarbons,less than 100 wt·ppm hydrolysable chlorine and less than 40 wt·ppm acidis obtained by preconcentrating the crude TDI solution to a solventcontent of <20%, followed by fractionation in a dividing walldistillation column. In U.S. Pat. No. 6,900,348, or in the correspondingEP 1 187 808, it is described that lighter-coloured diphenylmethanediisocyanates can be obtained by using phosgene having a bromine contentof <50 ppm. EP 0 816 333 claims a method for reducing the colour of TDIby treating the crude solution with hydrogen before the solvent isseparated off.

Special pretreatment of the toluylenediamine (TDA) used to produce TDIcan also lead to improved purity of the TDI. For example, EP 1 864 969claims a method for producing lighter-coloured TDI in which the TDA usedtherefor in the phosgenation comprises less than 0.1 wt. % alkylatedcyclic ketones, based on 100 wt. % TDA. In U.S. Pat. No. 5,872,278 orthe corresponding EP 0 866 057, a method is described in which the amineused is treated with solids containing Lewis and/or Brönstedt acidcentres before the reaction with phosgene. The isocyanates obtained thenhave a lighter colour than isocyanates produced using untreated amine.

Although these comparatively very complex methods permit the productionof TDI grades with greater purity and a lighter colour, there is noindication therein of which secondary components are responsible for theincreased catalyst consumption and the still insufficiently preventablediscolouration of the polyisocyanates and of the coatings producedtherewith, and how this discolouration can be prevented to a sufficientdegree. There is therefore still an urgent need for light-colouredaromatic coating polyisocyanates which are stable to ageing.

The object of the present invention was, therefore, to find a methodwith which the content of stopped catalysts in the polyisocyanates canbe reduced sufficiently that lighter-coloured polyisocyanates andcoatings which are more resistant to ageing can be produced therewith.

It has been possible to achieve that object with the method described ingreater detail below.

The invention is based on the surprising observation that catalystconsumption in the production of polyisocyanurates comprising isocyanategroups, based on toluylene diisocyanate can be reduced significantly iftoluylene diisocyanate having a content of2-chloro-6-isocyanato-methylcyclohexadienes (CIMCH) of <5 wt·ppm is usedfor their production. By means of the light-coloured polyisocyanuratesproduced by this method it is possible to produce lighter-colouredcoatings which are more resistant to ageing. Light-coloured in thiscontext means that the polyisocyanates so produced have APHA colourindices of <100 Hazen, preferably <75 Hazen, particularly preferably <55Hazen, measured on the basis of DIN EN 1557.

CIMCH can be in the form of 3 double bond isomers which can be presentin the TDI in different ratios. These are formed, for example, in TDIproduction from 1-amino-2-methyl-cyclohexenone contained in the TDAused, which in turn can form in the production of TDA fromdinitrotoluene (DNT) by partial nuclear hydrogenation of TDA andreplacement of an amino functional group by water. It is also possiblethat the keto functional group is already introduced proportionately byoxidative attack in the production of DNT by nitration of toluene, therefirst being formed nitrocresols which can then form the above-described1-amino-methyl-2-cyclohexenone in the subsequent hydrogenation.

Accordingly, the invention provides a method for producingpolyisocyanates comprising solvent and/or diluent and isocyanurategroups, based on 2,4- and/or 2,6-toluylene diisocyanate, having acontent of monomeric diisocyanate of <0.5 wt. %, based on polyisocyanateplus solvent, by trimerising

-   -   A) from 20 to 80 wt. % of industrially available mixtures        comprising substantially 2,4-toluylene diisocyanate and        2,6-toluylene diisocyanate comprising from 65 to 95 wt. %        2,4-toluylene diisocyanate and from 5 to 35 wt. % 2,6-toluylene        diisocyanate in the presence of    -   B) from 20 to 80 wt. % of solvents and/or diluents and    -   C) phenolic catalysts comprising dialkylaminomethyl groups        at a temperature of from 40 to 120° C. to almost complete        conversion, and then deactivating the catalyst by addition of        acid-reacting substances or by reaction with alkylating agents,        characterised in that        the toluylene diisocyanate used has a content of        2-chloro-6-isocyanato-methylcyclohexadienes (CIMCH) of <5        wt·ppm.

Almost complete conversion means that the TDI used is reacted to aresidual monomer content of <0.5 wt. %, based on polyisocyanate plussolvent.

The invention also provides polyisocyanates comprising solvent and/ordiluent and isocyanurate groups produced by this method, as well as theuse thereof as the polyisocyanate component in polyurethane coatings, inparticular in two-component polyurethane coatings.

There come into consideration as the toluylene diisocyanate A) inparticular 2,4-toluylene diisocyanate and commercial mixtures thereofwith up to 35 wt. %, based on the mixture, of 2,6-toluylenediisocyanate, which have a content of2-chloro-6-isocyanato-methylcyclohexadienes (CIMCH) of <5 wt·ppm. SuchTDI grades can be obtained, for example, by purposive removal of2-chloro-6-isocyanato-methylcyclohexadienes from the preconcentratedcrude TDI solutions by distillation in a dividing wall distillationcolumn, as is described in EP 1 413 571 B 1. Particular preference isgiven, however, to toluylene diisocyanates which are produced by gasphase phosgenation of TDA and whose content of2-chloro-6-isocyanato-methylcyclohexadienes is below the detectionlimit. Toluylene diisocyanate of such a grade is obtainable, forexample, from Bayer Material Science AG from the production at theCaojing site in China.

Two independent analytical methods have been used for the clearcharacterisation of the component2-chloro-6-isocyanato-methylcyclohexadienes. By means of gaschromatography techniques, different toluylene diisocyanate gradeshaving a 2,4 content of about 80 wt. % were tested for theirdissimilarities in the secondary component spectrum. By subsequentcoupled gas chromatography-mass spectroscopy, a molecular weight of 169g/mol was allocated to the three hitherto unknown compounds (CIMCHincluding two isomers). It was possible to obtain further structuralinformation from the fragmentation in a manner known to the personskilled in the art. By means of complex nuclear resonance spectroscopyexperiments (¹H-NMR, ¹H-COSY, ¹H-,¹H-TOCSY and ¹H-,¹³C-HMBC), thestructures indicated below could be allocated to the three componentswith m/z 169.

By purposive method development it was possible to set the detectionlimit of the isomers of CIMCH by means of gaschromatography-spectroscopy, using an Optima 5 HT column (60 m length,0.25 mm inside diameter, 0.25 μm film thickness) from Macherey-Nagel inan HP Series 6890 gas chromatograph from Hewlett Packard, at 1 wt·ppm.

As solvents B) there can be used diluents and solvents conventional inpolyurethane chemistry, such as, for example, toluene, xylene,cyclohexane, chlorobenzene, butyl acetate, ethyl acetate, ethyl glycolacetate, pentyl acetate, hexyl acetate, methoxypropyl acetate,tetrahydrofuran, dioxane, acetone, N-methylpyrrolidone, methyl ethylketone, white spirit, higher substituted aromatic compounds as areavailable commercially, for example, under the name Solvent Naphtha®,Solvesso®, Shellsol®, Isopar®, Nappar® and Diasol®, heavy benzol,tetralin, decalin and alkanes having more than 6 carbon atoms,conventional plasticisers, such as phthalates, benzoates, sulfonic acidesters and phosphoric acid esters, and mixtures of such diluents andsolvents. The concentration of the diluent and solvent is adjusted tofrom 20 to 80 wt. %, preferably from 40 to 60 wt. %.

Also suitable as solvents B) are polyisocyanates based on aliphaticdiisocyanates as are described, for example, in DE-A 4 428 107. Dilutelow-monomer TDI trimerisates which do not comprise readily volatilesolvents and diluents are obtainable therewith.

As phenolic catalysts C) for initiating and accelerating thetrimerisation reaction there come into consideration as Mannich basesspecific systems having a so-called negative temperature effect, whichlead to a selective incorporation of TDI even at higher temperatures.Such catalyst systems have N,N-dialkylaminomethyl groups and phenolic OHgroups bonded to aromatic systems. The alkyl groups are different oridentical radicals each having up to 18 carbon atoms, which areoptionally separated by oxygen or sulfur, or bridging alkyl groups inthe form of an alkylene group having up to 18 carbon atoms optionallycontaining oxygen or sulfur. The N,N-dialkylaminomethyl groups and thephenolic OH groups can be distributed on a plurality of molecules orpositioned on one or more benzene aromatic systems. Preferably,compounds that comprise both hydroxyl groups and dialkylaminomethylgroups in one molecule are used as catalyst systems.

Particular preference is given to the use of systems whosedialkylaminomethyl groups are in the ortho-position relative to aromatichydroxyl groups, the alkyl groups being identical or different C1- toC3-alkyl radicals.

The synthesis of suitable Mannich bases is described, for example, in DE25 51 634 A1 and WO 2005 70984 A1. Mannich bases which are preferably tobe used are those based on phenol, p-isononylphenol or bisphenol A,which are obtained by reaction with dimethylamine and formaldehyde, forexample according to DE-A 2 452 531 or Synth. Commun. (1986), 16,1401-9. Mannich bases based on phenol or bisphenol A are particularlypreferred.

The catalysts C) are used in the form of the pure substance or insolution optionally in a plurality of small portions or continuously. Intotal, from 0.1 to 0.8 wt. %, preferably from 0.3 to 0.6 wt. %, catalystis used for the production.

The trimerisation reaction according to the invention is carried outaccording to known methods, as are described, for example, in WO 200570984 A1.

The trimerisation is carried out in the presence of the solvent and/ordiluent component B). The trimerisation reaction takes place in thetemperature range of from 40 to 120° C., preferably from 50 to 70° C.The reaction time is generally from 5 to 48 hours, preferably from 10 to24 hours. If the content of free TDI in the reaction mixture is below0.5 wt. %, the trimerisation is terminated by thermal decomposition ofthe catalyst or, preferably, by addition of a catalyst poison. Suitablecatalyst poisons are protonic acids such as dibutyl phosphate oracylating and alkylating agents such as isophthalyl acid dichloride ortoluenesulfonic acid methyl ester.

Surprisingly, less catalyst is consumed in the method according to theinvention, with from 0.1 to 0.8 wt. % catalyst, than when conventionalTDI is used, so that lighter-coloured polyisocyanates which are morestable to yellowing are obtained.

Furthermore, when the trimerisation reaction is complete, a furthermodification of the product with low molecular weight or/and polymericcompounds comprising hydroxyl groups can take place.

The polyisocyanates produced by the method according to the inventionare preferably used in the production of coating materials which can becured under the effect of atmospheric moisture. They can likewise beused in the production of adhesion promoters, adhesives, printing inks,sealing materials and polyurethane moulded articles. They areparticularly preferably used as crosslinkers in 2-component systems withisocyanate-reactive compounds known per se. These include, for example,hydroxy-functional polyethers, polyesters, polyamides, polycarbonates,polyacrylates, polybutadienes or mixed types of the mentionedhydroxy-functional polymers. Low molecular weight diols and polyols,dimer and trimer fatty alcohols as well as amino-functional compoundscan also be used in 2K systems. Using blocked isocyanate-reactivecompounds, one-component systems can also be formulated; likewise, theproducts produced by the method according to the invention can also beused in blocked form as or in coating materials. Drying thereby takesplace at higher temperatures up to about 200° C.

In addition to the products according to the invention, there can alsobe used in the coatings other auxiliary substances and additives suchas, for example, conventional wetting agents, flow agents, anti-skinningagents, antifoams, solvents, mattifying agents such as, for example,silica, aluminium silicates and high-boiling waxes, viscosity-regulatingsubstances, pigments, colourants, UV absorbers, stabilisers againstthermal or oxidative degradation.

The coating materials obtained can be used in the coating of any desiredsubstrates such as, for example, wood, plastics, leather, paper,textiles, glass, ceramics, plaster, masonry, metals or concrete. Theycan be applied by conventional methods of application such as spraying,spread coating, flood coating, pouring, dipping, roller coating. Thecoating compositions can be used in the form of clear coatings and alsoin the form of pigmented coatings.

The coatings produced from the products according to the invention cureat 20° C. generally within a period of from several minutes to hours toform high-quality coatings. Curing can, however, also take place atlower temperatures (down to −5° C.) or in an accelerated manner athigher temperatures up to 200° C.

EXAMPLES

In the examples which follow, all percentages are by weight. Thefollowing methods were used to characterise the products obtained:

The NCO content of the resins described in the examples and comparativeexamples was determined by titration according to DIN EN ISO 11 909.

The dynamic viscosities were measured according to DIN 3219 with a DINmeasurement body 125 at 23° C. using a Reolab QC viscometer from AntonPaar in the shear rate range of from 1 to 1600 l/s.

The residual monomer contents were determined by gas chromatographyaccording to DIN EN ISO 10283.

The solids content (non-vaporisable portion) was determined according toDIN 3251 under the test conditions described therein for isocyanates.

The colour indices were measured at 23° C. on the basis of DIN EN 1557using a LICO 400 from HACH Lange in 50 mm disposable rectangularcuvettes.

Example 1 (in Accordance with the Invention)

426.0 g of a commercial mixture of 2,4- and 2,6-toluylene diisocyanatein the ratio 4:1 and having a content of CIMCH of 4.8 wt·ppm are placedtogether with 436.5 g of butyl acetate in a 1000 ml double jacketedground glass vessel flushed with nitrogen. The reaction mixture isheated to the desired reaction temperature of 75° C. The trimerisationreaction is started by the continuous addition of 7.5 g/h of a 30 wt. %activator solution (xylene) of a Mannich base based on bisphenolA/formalin/dimethylamine. When the trimerisation reaction has noticeablystarted, the continuous metered addition of activator is interrupted inorder safely to dissipate the reaction energy that is liberated. Whenthe desired reaction temperature is reached again, the metered additionof activator is continued. After about 7.5 hours, the entire amount ofactivator solution of 21 g has been metered in, and stirring is thencarried out for about 12 hours, while maintaining the previous reactiontemperature, until the desired NCO content is reached. In orderdefinitely to end the trimerisation reaction, the molar 1.15-fold amountof toluenesulfonic acid methyl ester (TSE) is added to the reactionmixture. After addition of 12.2 g of TSE, the reaction product isstirred for one hour at 80° C. The polyisocyanate comprising solvent andisocyanurate groups so obtained has the following characteristic values:

NCO content=7.97 wt. %

Viscosity=1290 mPa*s @ 23° C.

Residual monomer content=0.14 wt. %

Solids content=51.2 wt. %

APHA colour index=73 Hazen

Example 2 (in Accordance with the Invention)

The procedure is analogous to Example 1, but the reaction is carried outwith toluylene diisocyanate having a 2,4 content of about 80 wt. % whichwas produced by gas phase phosgenation, and a content of CIMCH below thedetection limit (<1 wt·ppm). In order to achieve the desired NCOcontent, only 15.3 g of the identical 30 wt. % activator solution arerequired. An amount of TSE of 7.6 g is used for the deactivation. Thepolyisocyanate comprising solvent and isocyanurate groups so obtainedhas the following characteristic values:

NCO content=8.06 wt. %

Viscosity=1240 mPa*s @ 23° C.

Residual monomer content=0.07 wt. %

Solids content=51.1 wt. %

APHA colour index=53 Hazen

Example 3 (not in Accordance with the Invention)

Using a toluylene diisocyanate grade having a 2,4 content of about 80wt. % which has a content of CIMCH of 160 wt·ppm, the procedure isanalogous to Example 1, but 33.6 g of the 30 wt. % activator solutionmentioned in Example 1 are required for the desired reaction conversion,whereby the deactivation is carried out by addition of 19.6 g oftoluenesulfonic acid methyl ester. The polyisocyanate comprising solventand isocyanurate groups so obtained has the following characteristicvalues:

NCO content=8.03 wt. %

Viscosity=1150 mPa*s @ 23° C.

Residual monomer content=0.17 wt. %

Solids content=51.3 wt. %

APHA colour index=140 Hazen

Example 4 (not in Accordance with the Invention)

The procedure is analogous to Example 1, but the reaction is carried outwith toluylene diisocyanate having a 2,4 content of about 80 wt. %,which has a content of CIMCH of 60 wt·ppm. In order to achieve thedesired NCO content, an amount of 28.5 g of 30 wt. % activator solutionis required. The reaction mixture is deactivated by addition of 16.6 gof toluenesulfonic acid methyl ester. The polyisocyanate comprisingsolvent and isocyanurate groups so obtained has the followingcharacteristic values:

NCO content=8.03 wt. %

Viscosity=1300 mPa*s @ 23° C.

Residual monomer content=0.20 wt. %

Solids content=51.0 wt. %

APHA colour index=125 Hazen

Application Example: Yellowing of a Coating Formulation

The polyisocyanates from Example 1 and Example 4 are each mixed in theNCO/OH ratio 0.7:1 with a commercially available polyester polyol(®Desmophen 1300, commercial product from Bayer AG, hydroxyl content 4wt. %) to give a total concentration of 40 wt. % in butyl acetate. Theformulations so obtained are applied wet to a standardised white platewith a layer thickness of 180 μm. The coatings so applied are dried for7 days at room temperature, which is defined as yellowing time t=0. Whenthe 0 value has been determined, the unprotected plates are subjected tonatural weathering and measured again according to previously determinedtime intervals (see Table 1). All the measurements were carried out bymeans of a Color-Guide 45°/0° colorimeter (BYK-Gardner). The colourchanges are summarised in Table 1, the total colour difference isexpressed in the manner known to the person skilled in the art as □E*.

□E* □E* □E* □E* □E* Sample 7 days 14 days 1 month 3 months 6 monthsPolyisocyanate from Example 1 1.51 2.42 5.36 9.42 13.66 CIMCH 5 × 10⁻⁴wt. % Polyisocyanate from Example 2 1.51 2.41 5.30 8.71 12.42 CIMCH <10⁻⁵ wt. % Polyisocyanate from Example 3 1.62 2.85 6.92 15.11 15.02CIMCH 1.6 × 10⁻² wt. % Polyisocyanate from Example 4 1.58 2.76 6.5812.86 24.78 CIMCH 6 × 10⁻³ wt. %

The application examples show that the coating formulations producedusing the polyisocyanates according to the invention from Examples 1 and2 have significantly greater colour stability.

The invention claimed is:
 1. A method for producing polyisocyanatescomprising solvent and/or diluent and isocyanurate groups, based on2,4-toluylene diisocyanate, having a content of monomeric diisocyanateof <0.5 wt. %, based on polyisocyanate plus solvent, comprisingtrimerising A) from 20 to 80 wt. % of 2,4-toluylene diisocyanate in thepresence of B) from 20 to 80 wt. % of solvents and/or diluents, and C) aphenolic catalyst comprising dialkylaminomethyl groups at a temperatureof from 40 to 120° C. to almost complete conversion, and thendeactivating the catalyst by addition of an acid-reacting substance orby reaction with an alkylating agent, wherein the toluylene diisocyanatehas a content of 2-chloro-6-isocyanato-methylcyclohexadienes (CIMCH)below the detection limit of 1 wt. ppm and wherein the polyisocyanatescomprising solvent and/or diluent and isocyanurate groups have APHAcolour indices <100 Hazen.
 2. The method according to claim 1, whereinthe toluylene diisocyanate is produced by gas phase phosgenation.
 3. Themethod according to claim 1, wherein the catalyst is obtained as aMannich base based on phenol or bisphenol A by reaction withdimethylamine and formaldehyde.
 4. The method according to claim 1,wherein the catalyst is deactivated by dibutyl phosphate orp-toluenesulfonic acid methyl ester.
 5. The polyisocyanates produced bythe method according to claim 1 comprising solvent and/or diluent andisocyanurate groups.
 6. A method comprising utilizing the polyisocyanateproduced according to the method of claim 5 as the polyisocyanatecomponent in polyurethane coatings.
 7. The method according to claim 6wherein the polyisocyanates comprise a crosslinker in two-componentpolyurethane coatings.
 8. A polyurethane adhesive comprising thepolyisocyanates produced according to the method of claim 5.